The invention relates to a communication network comprising several stations, at least one station being provided with a data processing unit and a receiver module for receiving the frames transmitted via the network, the receiver module comprising an electronic determination-type filtering device for filtering the frames, each frame comprising at least a header, the filtering device comprising a localizing device for localizing a group of bits in a received frame, the filtering device also comprising an identification unit which is connected between an output of the localizing device and a first input of the data processing unit, the identification unit comprising a memory and a comparator for identifying the frames destined for the station by comparing the contents of the group of bits localized in each frame with at least one reference value stored in the memory and by forming a result signal for each frame when this group exhibits the necessary correspondence with the reference value, the data processing unit being provided for receiving the frames validated on beforehand in function of the result signal.
Such a network is known from the article "An introduction to local Area Networks" by D. D. Clark, K. T. Pogran and D. P. Reed, published in Proceedings of the IEEE, Vol. 66, No. 11, November 1978, pp. 1497-1517. The electronic filtering device enables the station to accept only those frames circulating in the network which are destined for this station. Therefore, in a frame received, the localizing device localizes either a part, for example the most significant bits, or the entire section formed by the address bits which are part of the header. The bits set apart among the address bits are then compared in the identification unit which comprises a comparator and a memory for this purpose. One or more reference values are stored in this memory. When the bits set apart among the address bits are equal to one of the reference values stored in the memory, the identification unit forms a result signal which is applied to the data processing unit. The formation of a result signal signifies that the frame received is destined for the station and the frame is thus accepted by the data processing unit under the control of the result signal. The station is thus constructed to determine which of the frames circulating in the network it will accept.
Nowadays, communication networks are well known and fully developed considering the progress made in the electronics technology.
One of the development trends leads to a sub-division and the distribution of the tasks, for example, in local area networks (LAN), switched networks (telephony) and the transmission in packets (TRANSPAC), be it private or public, satellite telecommunication networks.
The general term "broadcast network" is often used, this term means that all messages transmitted on the transmission carrier of the network by any system connected thereto are received by all systems connected. This mode of operation not only avoids complications concerning the physical routing of the messages (direct communications), but also facilitates, on the one hand, the implementation of communication systems between the logic entities and, on the other hand, the implementation of an efficient management system for the network, and also enables monitoring of the traffic from any point of the network.
However the flexibility offered by broadcast networks also has its drawbacks. The storage and processing capacity of each station is readily exceeded by the multitude of messages received, because the messages which are not intended for the station must also be examined by the electronic filtering device and the data processing system in order to ensure that they are only deliberately ignored.
Therefore, there are actually two types of filtering devices: one which corresponds to the physical addressing mode, where the station recognizes only a single physical address (a single address word stored in the first memory,) and one which corresponds to the logic addressing mode, where the station recognizes several logic addresses, for example, the subscribers to a service, or the participants in a telephone conference; all messages transmitted by each entity concerned contain the same logic number which is referred to as the logic address of the service or the telephone conference, and each station connected to the same logic number has to filter all messages thus addressed thereto in the broadcast mode; each station must be capable of simultaneously recognizing several logic numbers.
A drawback of known electronic filtering devices in a station which forms part of a communication network, consists in that the localizing device is only capable of localizing a well-defined part of the address bits of a frame; moreover, this well-defined part is the same for each frame received. Thus, the known localizing devices offer hardly any flexibility as regards the selection of the parts to be bounded in the address bits. Moreover, the memory of the identification unit is a read-only memory which means that it is impossible to modify, as the case may be, the contents of this memory for each frame. Due to these limitations, the filters now implemented at the level of the access control layer, perform only a probability-type filtering operation by means of standard components of the modulo-type (hash-coding) and the filtering thus performed, therefore, is not perfect and leads to undesirable overloading of the station, which in turn, may lead to the loss of the message or information when this logic addressing method is intensively used.